Tunable sources of coherent radiation have been sought ever since the development of the laser, and work continues at developing new devices to provide a broader range of tunability, to provide continuous or nearcontinuous tuning over the range covered, or to provide better economy.
One approach used in the prior art is to extend the range of a tunable laser, such as a dye laser, toward the infrared by use of the Raman-scattering process in which one or more of a series of frequencies are produced in a single pass through a nontunable Raman-active medium such as benzene. These frequencies, called Stokes lines, have energies which are less than the energy of the generating laser beam and, in the prior art, are separated equally in energy. When the frequency of the tunable generating laser is changed, the frequencies of the Stokes lines change by an equal amount. Optical fibers were not employed by the prior art.
One disadvantage of this prior art method is that a reasonably large Raman energy shift, which is required in order to extend the frequency range covered by the device, may be greater than the tuning range of the generating laser, so that there is a gap between Stokes lines which is not accessible. Another disadvantage of this method results from the fact that the Stokes lines are equally spaced in frequency. For applications which require photons of two different energies, such as sum and difference frequency mixing to generate ultraviolet or infrared radiation, a single prior art Raman generator will be unable to supply beams of the correct energy because the several frequencies produced track the generating laser beam and are not independently adjustable. In that case, two frequency generators would be required to provide beams of the right frequencies.
Another prior art device is the fiber-optic Raman oscillator, which differs from the Raman generators mentioned above in that it is an oscillator, in which the light makes many passes through the Raman medium, and in that the Raman medium is capable of responding over a range of frequencies. Such an oscillator is disclosed in U.S. Pat. No. 3,705,992, dated Dec. 12, 1972, issued to E. P. Ippen et al, which patent is incorporated herein by reference. The use of a tuning element incorporated in the oscillator cavity (FIGS. 3 and 4 of the Ippen et al. patent) is suggested for tuning the oscillator frequency within a range of about 100 Angstroms (using a fixed-frequency pump). This device required a great deal of power (having a threshold for Stokes oscillation of 500 watts in the fiber (Col. 3, line 43)). It employed a pulsed laser, in part to achieve the high peak power that was required. The device was incapable of oscillating at more than one frequency even if sufficient power to reach the higher thresholds were available because of the structure of the device.
It was known in the prior art on the basis of single-pass measurements that production of Raman radiation in glass is weaker by a factor of more than 100 than it is for standard media such as benzene but the amount of intracavity Stokes power in a lossy cavity (having a factor of two attenuation in the fiber and another factor of two in the prisms) was not known to the prior art.
A phenomenon known to the prior art is that of superradiant emission in which Stokes radiation is generated on a single pass through a fiber, which radiation is concentrated in a subrange near the peak of the Raman gain curve that is small compared with the total Raman gain range of approximately 500 cm.sup.-1.